We are investigating the methylation of the epsilon-amino groups of lysine in the arginine-rich histones in vitro. When brain chromatin or nuclei from young rats were incubated with (methyl-3)-S-adenosyl-L-methionine, significant amounts of (3H)methyl were incorporated into the lysyl residues of histones H3 and H4. Methylation proceeds stepwise, resulting in a stable ratio of 0.17:1.0 for mono- and dimethylysine in histone H4 and 0.7:1.0:0.17 for mono-, di- and trimethyl-lysine in histone H3. Both in vitro and in vivo experiments indicate that the histones are methylated after they are bound to histones. The enzyme catalyzing these reactions is firmlY bound to chromatin. In the bound state it will not methylate free histones. The enzyme can be solubilized from chromatin by sequentially washing chromatin with distilled water. The enzyme has been partially purified. As yet we have no evidence for more than one enzyme. The enzyme loses specificity when assayed against free histones. All histone subfractions are methylated; however, histones H3 and H4 are still the best methyl acceptors. Further purification procedures and kinetic properties of the free and bound enzyme are presently under investigation. BIBLIOGRAPHIC REFERENCES: Duerre, J. A. and Ford, K.M. In vitro Studies on Histone Methylation in Rat Brain Nuclei. Fed. Proceedings, 1858, 1976. Wallwork, J.C. and Duerre, J.A. A Study of Histone Transmethylase from Rat Brain. N. Dak. Acad. Sci. Proceedings, 1976.